The present invention relates to a driving device to rotate a valve stem included in valves such as a butterfly valve, a ball valve and a gate valve, and, more particularly, to a driving device having a novel mechanism suitable as an electrically driven actuator utilizing an electrical power source.
Valves adapted to control the supply of various fluids are employed in the chemical industry and other industries as important parts of various equipment. The manner of use of these valves has recently become automatic as a marked tendency to meet the automation of the equipment with which they are associated. The power source for the automatic control system in such equipment may be a pneumatic source, a hydraulic source or other source such as an electric power source, as the individual case demands.
The electrically driven actuator of the prior art using electric power as a drive source has been disadvantageous in that the equipment is generally bulky, of heavy weight, of high cost and of low efficiency. In such prior art electrically driven actuators, a decelerator comprising a worm gear mechanism has often been used and, accordingly, an electric motor has been coupled to a valve stem always at a constant reduction ratio. On the other hand, the output torque characteristic of the electric motor has largely depended upon the rotational velocity of the electric motor and a torque from r.p.m. of zero, i.e., a starting torque has had to be sufficiently large to start a driven apparatus. Accordingly, selection of the electric motor has been based not only on the rated torque thereof at the normal condition but also on the magnitude of the starting torque.
In view of the foregoing, the prior art mechanisms in which, as previously described, the electric motor is coupled to the valve stem at a constant reduction ratio without any consideration of the high torque required to start the rotation of the valve stem, have been ineffective to obtain a desired operation. In rotational operation of a valve, a slow velocity is desirable in opening or closing the valve so as to eliminate so-called surged pressure possibly generated within tubes having the valve connected thereto. During normal operation, however, fast velocity is desirable. So it is often necessary to provide the actuator with a velocity controller. However, prior art controllers have been generally complex in construction and difficult to operate. The various disadvantages previously pointed out have resulted from this deficiency.
Although it has already been proposed that a drive transmission to obtain a final rotary drive be achieved at two points symmetrical with respect to the rotary shaft so as to provide a mechanism of small size and light weight, no satisfactory result has been obtained heretofore in terms of a mass production mechanism. This is true since the effect of the prior art drive transmission has often been biased to any one of the two points due to the unevenness of working and assembling accuracies of the respective parts.
A principal object of the present invention is, therefore, to overcome the foregoing disadvantages by providing a novel device to open and close a valve stem and, more particularly, by providing a driving device which is suitable as an electrically driven actuator utilizing an electric motor as a drive source and operating at an angle of 90.degree..
Another object of the present invention is to provide an improved actuating mechanism comprising a new characteristic of rotational velocity in itself adapted for effective valve operation.
Another object of the present invention is to provide an electrically driven actuator of small size, low cost and light weight and thereby not only facilitate the incorporation of an electrically driven actuator into an automatic valve, but also simplify the design of various apparatuses of which the automation is desired and where it is also desired to use an electric power as a drive source in a control system of the apparatus. For example, the actuator of the present invention is especially advantageous in applications where there are a plurality of driven mechanisms spaced from each other and a pneumatic or hydraulic drive is not suitable or where a pneumatic or hydraulic drive source is not advailable or difficult to install.
A further object of the present invention is to permit such a mechanism to be employed to provide a drive transmission having a final rotary drive achieved at two points symmetrical with respect to a rotary shaft.
Further objects of the present invention will be apparent from reading the following description.
These objects are achieved, in accordance with the present invention, by the drive device to open and close the valve stem, which includes therein a mechanism as described below.
The mechanism is principally characterized by the provision of a threaded shaft arrangement which drives a transversely disposed rotary shaft adapted to be operatively connected to a valve apparatus to be actuated. The threaded shaft arrangement is itself adapted to be driven by an electric motor and apply a uniform rotational drive to the rotary shaft.
In one of the illustrated embodiments, the novel driving device to open and close the valve stem disclosed here according to the present invention is an electrically driven actuator comprising a housing including a rotary shaft adapted to connect the actuator to a valve apparatus. The rotary shaft extends transversely of an axis of the housing within the housing and has at least one end projecting outwardly from the housing so that the one end may be associated with a valve stem. Preferably, a pair of threaded shafts extend in parallel relationship with respect to each other within the housing in a plane transverse to the rotary shaft, and the threaded shafts are located symmetrically with respect to the rotary shaft. Movable bodies including projecting shafts are mounted on the respective threaded shafts so that these movable bodies may move along their respective theaded shaft as the threaded shafts rotate. Operating arms which are shaped symmetrically with respect to the rotary shaft are fixed thereto within the movement ranges of the projecting shafts so that the operating arms may swing integrally with the rotary shaft. Longitudinal slots or slits are formed in the operating arms for slidably receiving the respective projecting shafts of the movable bodies. The directions in which the threads of the respective threaded shafts are wound and the mechanism by which a drive force is transmitted from the electric motor to the respective threaded shafts are so arranged that both movable bodies may be moved in opposite directions at an equivalent velocity as the electric motor rotates.
A mechanism is provided to adjust, absorb and combine any relative phasal shift between the threaded shafts. Preferably, the mechanism comprises a wide groove formed in the inner periphery of a wheel adapted to be engaged with one of the pair of threaded shafts and to thereby transmit the drive force from the electric motor to the one threaded shaft so that the relative positions of the one threaded shaft and the wheel may be adjusted according to a position at which a pin is urged into the groove.